Chandra and HST Observations of Radio-Selected (Wandering) Massive Black Hole Candidates in Dwarf Galaxies

Abstract

We present Chandra X-ray Observatory and Hubble Space Telescope (HST) follow-up observations of 12 dwarf galaxies from Reines et al. (2020) that are potential hosts of radio-selected active galactic nuclei (AGNs), eight of which are non-nuclear and possible ``wandering" black holes (BHs). Our multi-wavelength analysis indicates a heterogeneous sample with five radio sources detected at both X-ray and optical wavelengths within positional uncertainties and non-detections for the remaining objects. Of the radio objects detected in the X-ray/optical, three have multi-wavelength evidence for hosting nuclear massive BHs and one object is consistent with an extreme compact starburst. Only one of the off-nuclear radio sources has a significant optical counterpart and we present Palomar spectroscopy that identifies this object as a background AGN. We cannot definitively determine if the seven remaining off-nuclear radio sources are wandering massive BHs in the target dwarf galaxies or background AGNs, although the three sources with the largest offsets have compact radio cores detected with the Very Large Baseline Array and are consistent with expectations for background AGNs (Sargent et al 2022). Our HST sensitivity limits also allow for wandering massive BHs in the target dwarf galaxies that are hosted by stellar clusters with masses $\lesssim 10^6 M_\odot$.

Figures (4)

• Figure 1: Chandra 0.5-7 keV images for our targets galaxies with X-ray detections. The red cross shows the locations of the detected compact radio sources. While the centroids of the Chandra sources do not always directly align, the sources are roughly positionally consistent.
• Figure 2: Palomar Double Spectrograph (DBSP) spectrum of the bright optical counterpart to the radio source in ID 64. Foreground narrow emission lines in the dwarf galaxy are marked in black and correspond to a redshift of $z=0.034$. There is another set of emission lines shown in red that correspond to a redshift of $z=0.761$ and that likely originate from a background AGN associated with the radio source.
• Figure 3: Three-color HST images of our galaxies. Green corresponds to the wide $I$-band F814W filter or wide $YJ$ F110W filter (for ID 26), red corresponds to the H$\alpha$ F680N or F665N filter (for ID 6) or wide $V$-band F606W filter (for ID 26) and blue corresponds to the wide $B$-band F475W or SDSS $g$-band F438W filter (for IDs 64, 77 and 92) or wide UV F275W filter (for ID 26). The white/black circles have radius 025 and show the location of the compact radio source. The yellow circles show the location of the X-ray detections with radii 05. The red circles indicate the location of the SDSS fibers with diameter of 30. IDs 26, 64, 82, 83 and 92 all have both optical counterparts to the radio sources observed in all filters and X-ray detections corresponding to roughly same sky location (also see Figure \ref{['fig:xray_detections']}).
• Figure 4: Observed Chandra X-ray spectra with sufficient counts for spectral modeling, best fit power law model (orange line) and ratio residuals. The data have been rebinned for plotting. The simple power law model with fixed Galactic absorption describes IDs 64, 83, and 92 well, but the spectral shape of ID 26 hints at the presence of a feature at $\sim 0.9$ keV.